In the end, this particular question might be striking at the heart of Fitzgerald's work. There is a strong emphasis on the role of the individual and their relation to the social order. In a larger sense, the issue here might be the role of blame as being something present in the social order, but having to be shouldered by the individual. Fitzgerald does go to great lengths to show that the social order in which Gatsby is immersed as one that is predicated upon unhealthy or unsound emotional practices. In this light, individuals are placed in a setting where people do act badly. However, it is up to the specific individual to make a conscious choice to continue this pattern or step away from it. Gatsby is caught up in this world in his hope of winning over Daisy. In the end, while the world itself is fairly rotten, he has to bear the brunt of it for participating in it in the manner and to the extent that he did.

"They were careless people, Tom and Daisy." I think this quote says it all. So I don't agree with the statement. Yes, Gatsby was guilty of making an idol out of Daisy:

Almost five years! There must have been moments even that afternoon when Daisy tumbled short of his dreams—not through her own fault, but because of the colossal vitality of his illusion. It had gone beyond her, beyond everything. He had thrown himself into it with a creative passion, adding to it all the time, decking it out with every bright feather that drifted his way. No amount of fire or freshness can challenge what a man will store up in his ghostly heart.

He built up his "empire" mainly to impress and win her. He thought they could go back to the way things were five years earlier when he was a soldier and she was his girl. But what he didn't figure on was that careless Daisy had married even more careless Tom. If Tom had not been messing around with Myrtle, she would never have run out in front of the car, she would not have been killed. And don't forget that it was Daisy that was driving the car that killed Myrtle. George, Myrtle's husband, blamed Gatsby for running down his wife and shot Gatsby. So I think that Tom and Daisy are responsible for Gatsby's death.

That said, Gatsby himself is responsible for the tragedy of his life because he was so unrealistic in the object of his love. He stored up a "ghostly" memory of Daisy in his heart. But he was destroyed by the "careless" characters.

I know lots of people will disagree with this by saying that Gatsby should have told everyone that Daisy was driving the car. If he had done this, George would not have shot him, so he did cause his own destruction. However, if not for Tom's cheating ways, there would not have been a Myrtle and George.

Concerning the issue you ask about in The Great Gatsby, arguing absolutely, either way, is difficult. The novel, like most high quality, sophisticated fiction, is ambiguous. The book isn't a good guy and bad guy kind of book. Characters, like actual people, are mixtures of good and bad traits in the novel.

Tom tells Wilson that Gatsby owns the car that hits Myrtle. Wilson kills Gatsby. Tom is guilty. But it's not that simple.

Tom likely doesn't know that Daisy was actually driving the car. Daisy doesn't confess to him. Daisy guilty. But it's not that simple.

Gatsby plays a part in Daisy not telling Tom, and Gatsby certainly doesn't blame her. He lurks outside of her house all night in order to protect her from Tom. He still loves her. Is Gatsby guilty of his own murder?

One could go on and on in this way. You could argue that Wilson pulls the trigger, so he and he alone is responsible. Tom and Daisy certainly couldn't be charged with Gatsby's murder. They didn't put Wilson up to it.

And Gatsby's illusion is Gatsby's illusion. Is Daisy supposed to lie and pretend that she never loved Tom just to fulfill Gatsby's illusion? Gatsby tries to recapture a past that never was. He is on a quest that is doomed to fail. Yet, does that mean that he causes his own destruction?

In short, I suggest that your question is unanswerable. I suggest that to answer your question is to simplify an ambiguous novel that, in terms of its ambiguity, accurately reflects life.

But, I know you have an assignment to do, so, if I had to, I'd go with Gatsby causing his own downfall. Though he, like others in the novel, is a victim of Tom and Daisy, he is the foolish one who chases after a dream that never was. Daisy never loved him the way that he loves her. And when it comes down to it, he isn't satisfied with having Daisy love him now, in the book's present. She does love him and is ready to choose him. But that isn't enough. He has to maintain the illusion that their love is something special, that it is never ending, and that Daisy always loved him and never loved Tom. As Daisy tells him: he asks too much.